The present invention relates to metalized film laminates for packaging material applications which are corrosion resistant and are suitable for packaging food items.
In the field of packaging materials for food products, a metalized film layer has often served as a barrier against moisture and oxygen. For example, U.S. Pat. No. 5,096,630 to Bothe et al. disclose a method for producing a multilayered film which comprises a metalizable polypropylene surface for use with packaging food products. U.S. Pat. No. 5,153,074 to Migliorini discloses a film suitable for use as containers for foods comprising a metalized ethylene vinyl alcohol coated polypropylene layer which provides unexpected moisture barrier characteristics. Additionally, U.S. Pat. No. 5,206,051 to Theisen et al. disclose a process for producing an improved packaging film for food having low permeability to water vapor by metalizing the surface of a polypropylene/acid terpolymer blend. U.S. Pat. No. 5,728,467 to Watanabe et al. disclose a multilayered laminate having good gas barrier properties as a result of a film of metal formed on a surface of various plastic substrates.
Metalized film surfaces are susceptible to corrosion. This occurs when the metal comes in contact with water vapor from sources such as the environment, food items enclosed within the package or residual moisture introduced during the manufacturing process and/or from the materials used in the construction of the package. Among the common indications of corrosion manifested in metalized packaging films are oxidation, pitting, tarnishing, mottling, or discolorations. Corrosion of metalized film surfaces, such as metallic aluminum for example, can destroy the barrier properties of the package leaving it no longer useful as a packaging material for food. Uninhibited corrosion can be very destructive to both the package and its contents; resulting in actual material loss in the package, deterioration of the flavor and aroma components in food, and substantial decrease of the shelf-life of the product.
Corrosion inhibiting formulations for preventing corrosion of metal layers in plastic packaging materials have been reported. U.S. Pat. Nos. 5,209,869, 5,320,778, 5,344,589 and 5,422,187, all issued to Miksic, disclose vapor corrosion inhibitor/desiccant formulations. The inhibitors described in these four related patents are selected from formulations comprising anhydrous molybdates mixed with benzotriazole and sodium nitrite or from a formulation comprising amine benzoate, amine nitrates and benzotriazole. These patents disclose that the formulations can be incorporated into laminates containing a central metal layer. The film laminates find particular application in the formation of enclosures about metallic articles susceptible to corrosion, and provide a relatively dry corrosion inhibiting atmosphere therein.
Alternatively, U.S. Pat. No. 5,958,115 to Bottcher et al., discloses a method of producing corrosion-protective packaging materials in which one or more corrosion inhibitors are distributed into a metal oxide gel matrix which is modified with an organic polymer into a coating that is applied to paper or plastic. The corrosion inhibiting agent comprises phenols, hydroquinones, nitrites, organic acids, amines, amides, thiazoles, triazoles, imidazoles and mixtures thereof. The materials made according to this method are said to be suitable for producing corrosion-protective packaging materials for coating metallic or metalized articles.
Also, U.S. Pat. No. 6,033,599 to Lozano et al., discloses a corrosion inhibiting composition which includes a combination of inhibiting components comprising an alkali metal nitrite, an alkali metal benzoate and an alkali metal molybdate. The corrosion inhibiting compositions are incorporated within plastic wrapping materials in the form of laminated or blown films. The laminates and/or films are described as particularly useful for packaging corrodible metal articles.
It is of foremost importance when preparing corrosion-resistant metalized film laminates suitable material for food packaging applications, to consider the toxicity of the corrosion inhibiting agent. Many of the anticorrosion compounds and compositions described in the patents above contain nitrites, chromates, amines, phenols, hydroquinones, amides, thiazoles, triazoles, imidazoles and molybdates. Although these types of compounds are effective anticorrosion agents, it is known that chromates, phenols and combinations including amines and nitrites are deleterious to health. Furthermore, most of these compounds have not been approved for incidental food contact as required by law.